This invention relates to document readers, and more particularly to readers having sensing heads arranged to read both sides of a document.
In previous devices for reading marks on both sides of a document, it has been found desirable to have the document wipe directly against the mark sensing heads to ensure that all marks on the card are detected. One arrangement for accomplishing this result employs sensing heads with reading ends formed in a V-shape, each of the sensing heads having transmitting and receiving optical fibers which terminate along the apex of the "V". Radiation emitted from the transmitting fibers is reflected back to the receiving fibers by marks on the document to indicate the presence of a mark. The heads are positioned on opposite sides of the card path and mutually offset in the direction of document travel with their reading ends overlapping in a direction normal to the document surface. Passing documents are thereby slightly wrapped around the apex of each head and wipe against the apices during transit through the reader.
While the wiping contact between the document and sensing heads does perform its intended function of keeping the document on a precise transport path relative to the heads, a serious wear problem arises because of the abrasive nature of typical documents such as computer cards. The sensing head is typically formed from a plastic which wears away under repeated wiping by many cards. This exposes the optical fibers to abrasion by the cards, and the fibers also become worn. After initially reading perhaps 50,000 cards, it has been found that the sensitivity of the sensing heads is changed to such an extent due to wearing of the optical fibers that adjustment of a circuit which amplifies the reflected radiation signals is necessary. Thereafter the sensing heads continue to degrade as more cards are read, but at a slower rate, with adjustment being required every 100,000 to 250,000 cards or so. The adjustment rate depends on such factors as the surface characteristics and weight of the cards, the construction of the sensing heads, and the required reading accuracy. Eventually a point is reached at which the sensing heads are unusable and must be replaced.
In addition to the wear problem, repeated card wiping produces a carbon buildup on the optical fibers by transferring carbon from the card marks to the fibers. This reduces the reading capability with respect to both the amount of radiation that can be directed onto a card, and the amount of reflected radiation that is sensed by the heads.